The proposed research aims to purify the activation fragment of the fourth component of human complement, C4a. Purified C4a will be used to raise monospecific antibodies, which in turn will be used to develop a quantitative competitive inhibition radioimmunoassay for C4a. Plans have been put forth to apply the radioimmunoassay for C4a activation in "immune-complex" positive diseases. This project also aims to delineate the mechanisms of action responsible for the binding of fragment C4b to biological membranes and immune aggregates; responsible for inactivation of the hemolytic activity of C4 by amines; and responsible for apparent autolytic cleavage of C4 that is inhibited by methylamine. Finally, this proposal details a plan to sequence the primary structure of the newly exposed N-terminus of fragment C4b.